With the development of processing capabilities of processors such as digital signal processors (DSPs), it has become possible to reconstruct spatial expansion at the time of acoustic listening using a headphone by convoluting an audio signal with a head-related transfer function (HRTF).
For example, Patent Literature 1 discloses a method of improving space perception in virtual surround to prevent reproducibility of a front channel from being damaged while improving reproducibility of surround channels by a pair of loudspeakers. Further, Patent Literature 2 discloses a technique for localizing an audio image outside the head of the user through an audio signal convoluted with an average HRTF.